The term "television receiver" as used herein means a device for receiving and processing television signals, and thus includes television receivers having a display system (commonly referred to as television sets) and also devices without a display system such as videocassette recorders.
The term "television monitor/receiver" as used herein means a television receiver having an RF tuner and baseband signal input circuitry.
Many modern television receivers include picture-in-picture (also known as "pix-in-pix", or "PIP") capability. In a PIP system, video signals from two different sources may be combined to form a single image for display on a display screen wherein the combined image comprises a main picture, and an inset smaller picture usually located near one of the corners of the display screen.
For example, Sony videocassette recorder (VCR) SLV-70F has PIP capability wherein one signal source must be a videotape in playback mode, and the other signal source is selected from a group consisting of a built-in television tuner and two external sources of video. In the above-noted Sony system, the audio signal selected for reproduction is the audio signal associated with the picture displayed in the main viewing area. If the displayed positions of the video image signals from the two sources are interchanged (i.e. "swapped"), then the audio signal source associated with the video currently displayed as the main picture is automatically selected.
The Sony system employs a three-input audio selector switch having a single control input line. Unfortunately, this requires a trilevel switching signal (i.e., including low, half-high, and high levels) to be developed in order to select the proper audio source. The trilevel switching system undesirably requires both a more complex encoder and a more complex decoder, and fails to take advantage of the noise immunity inherent in bilevel digital switching circuitry.
A television monitor/receiver having multiple electronically selectable auxiliary baseband signal inputs is known from the RCA CTC-140 manufactured by Thomson Consumer Electronics, Indianapolis, IN. This monitor/receiver employs bilevel digital switching logic to automatically provide the proper audio when each of the video source selections is made (i.e. tuner, AUX1, or AUX2), rather than using a more complex trilevel switching arrangement. However, this monitor/receiver does not have PIP capability.